Wednesday, June 24, 2009
Blacktail Canyon is on river right at Mile 120. Unlike so many other side canyons, it does not end in a spectacular pool or a waterfall. What it does have is a sand and gravel beach, although not one most folks would recognize right away.
It is the base of the Tapeats Sandstone, lying on top of the schist at head level as we head up the canyon. This is the Great Unconformity - a boundary in the rock layers that represents a gap between incredibly old rocks and rocks that are merely ancient. The top of the underlying schist, perhaps 1.8 billion years old, is the eroded surface of a rocky coastal landscape caught in time half a billion years ago. As the ocean gradually rose, the waves planed off this surface, breaking off chunks of the metamorphic rock and spreading it in layers on the advancing beach. What we see now on the wall of Blacktail Canyon are multiple layers of sand separated by bands of coarse, angular gravel derived from the schist (there are quartz veins in the underlying rock and quartz clasts in the overlying beach deposits).
I suppose for each of the layers we see preserved, many more were formed and then erased. I wonder if each of these layers records a storm or a series of spring tides? Maybe on closer examination we would find thinner layers that represented the sediment moved by individual waves?
Imagine a rocky shoreline on the New England coast. The rocks are Paleozoic schists and phyllites, intruded by slightly less ancient granites. The Atlantic is at your feet, but the coast is gradually subsiding and the beaches advancing farther onto the land. With time, these rocks are buried by more layers of beach sand and then later, as the water deepens, with silt and mud and eventually limestone. Half a billion years later, the land has been uplifted, a river has carved a deep canyon, and some strange creature in a rubber raft discovers your fossilized cellphone (the size and basic skeletal structure of a 3" trilobite) on the unconformity that separates the schist from the beach.
The Tapeats was my favorite rock formation in the Canyon - partly because it told this wonderful story of ancient shorelines and partly because it was a distinctive and beautiful feature of the canyon. Downstream from Blacktail we float through Conquistador Aisle, with beautiful ledges of Tapeast Sandstone lining the river.